Application of Stem Cell Boost for Immune Effector Cell Associated Hematotoxicity (ICAHT) Following BCMA Directed Chimeric Antigen Receptor (CAR) T-Cell Therapy for Multiple Myeloma (MM)

Introduction: Persistent ICAHT is increasingly recognized as an adverse effect following BCMA CAR T-cell therapy. Alongside supportive care measures, we previously reported that stem cell boost helps ameliorate ICAHT. In addition, stem cell boost may minimize long-term complications such as infections while enabling therapeutic options in the post CAR T-cell therapy relapse setting. Herein, we report our follow-up of a multi-institutional experience with ICAHT in patients who received autologous BCMA CAR T-cell therapy both in commercial and clinical trial settings. Methods: We included recipients of autologous BCMA CAR T-cell therapy at 2 large academic centers between 2020 and 2024. For this study, neutropenia was defined as an absolute neutrophil count (ANC) of ≤ 1000, thrombocytopenia as platelet counts ≤ 50, 000 and anemia as hemoglobin ≤9 g/dL. ICAHT was characterized by the presence of any of these hematological abnormalities. The variables were assessed at 21 days, 3 months, and 6 months post CAR T-cell infusion. Application of stem cell boost was at the discretion of treating physician. Median follow up time for patients still under observation was 16. 5 months. Results: A total of 159 patients received BCMA CAR T-cell therapy, 69 were treated on a clinical trial, 65 received commercial idecabtagene vicleucel and 25 commercial ciltacabtagene autoleucel. The median age was 65 (IQR59-70) years, with 96/159 (60%) being male. 139/159 (87%) received at least one prior ASCT and the median prior lines of therapy was 5 (IQR 4-6). At D+21, ICAHT was observed in 58% (n=89/154) of patients, which reduced to 28% (n=39/137) and 17% (n=21/122) at the 3-month and 6-month assessments, respectively. Risk factors for ICAHT at D+21 were as follows: high risk disease by FISH or PET CT, use of tocilizumab therapy, prior ASCT, higher median prior lines of therapy, and ICANS. Among patients with ICAHT at D+21, 43% had persistent ICAHT at 3 months and 27% at 6 months. About 92% (n=82/89) of patients with ICAHT at D+21 had stem cells in storage and 27% of those (n=22/82) received a stem cell boost. The median time to stem cell boost following CAR T-cell infusion was 52 (IQR 30-143) days. The median dose of stem cell infused was 4 (IQR 2. 46-5. 60) x106 CD34/kg cells. As expected, patients with ICAHT at D+21 who received a stem cell boost had significantly worse cytopenia prior to the boost compared to those without stem cell infusions: hemoglobin levels were 8. 5 g/dL vs. 9. 5 g/dL, p 0. 002 and median platelet counts were 21 x 109/L vs. 48 x 109/L, p 0. 001, respectively. ANC levels were not significantly different between the groups (800/μL vs. 290/μL, p 0. 119). Stem cell infusion significantly improved hemoglobin levels at 3 and 6 months to 10. 9 g/dL, p 0. 001, and 12. 2 g/dL, p = 0. 002, respectively. Platelet counts at 3- and 6-months post stem cell boost also significantly improved to 127 x 10⁹/L, p 0. 001, and 152 x 10⁹/L, p 0. 001, respectively. ANC levels showed a statistically significant increase to 3000/μL at both 3 months (p = 0. 003) and 6 months (p = 0. 021). Overall blood counts were similar between the group with ICAHT at D+21 with or without a stem cell boost at 6 months, despite the statistically significant and profound cytopenia noted prior to stem cell boost in patients with ICAHT at D+21 who received a boost, compared to those who did not, suggesting a clinically meaningful effect of stem cell boost in patients with ICAHT. We further investigated whether the occurrence MDS/AML, post CAR T cell therapy correlated with ICAHT and/or stem cell boost. There were 4 cases of MDS and 1 case of AML post CAR T cell therapy. All 5 cases of MDS/AML occurred in patients who did not have ICAHT and/or a stem cell boost. The 12-month PFS was 66. 6% (95% CI 59. 2-74. 9) and OS was 88. 3% (95% CI 83. 3-93. 5). On multivariate analysis the presence of ICAHT at D+21 and use of stem cell boost were not associated with inferior PFS or OS, the presence of high-risk FISH and EMD on PET CT conferred inferior outcomes. Conclusion: About 58% of patients had ICAHT at D+21 post BCMA CAR T- cell therapy for MM. Nearly a third of these patients with ICAHT at D+21 had severe cytopenias and received a stem cell boost with clinically and statistically significant improvement in the cell counts at 3 and 6 months. The improvement in counts could help to prevent infections, reduce transfusion requirements and allow patients to undergo further therapy after progression from CAR T-cell therapy.